1. Field of the Invention
The present invention relates generally to a communication device and method for a CDMA communication system, and in particular, to a device and method for performing closed loop power control in a control hold state.
2. Description of the Related Art
A conventional Code Division Multiple Access (CDMA) mobile communication system based on the IS-95 standard primarily supports a voice service. However, a mobile communication system in accordance with the IMT-2000 standard will support not only the voice service, but also a high-speed data transfer service. For example, the IMT-2000 standard can support a high-quality voice service, a moving picture service, an Internet search service, etc.
In a mobile communication system, a data communication service is characterized by short transmissions (i.e., burst data) alternating with long non-transmission periods. Therefore, for the data communication service, a mobile communication system employs a channel assignment method in which a dedicated channel is assigned for only the short periods of (i.e., the burst duration) data transmission. That is, taking into consideration the limited radio resources, base station capacity and power consumption of a mobile station, the mobile communication system connects a traffic channel and a control channel only for an actual data transmission duration and otherwise releases the dedicated channels (i.e., the traffic channel and the control channel) when there is no data to transmit for a predetermined time. When the dedicated channels are released, communication is performed through a common channel, thus increasing the efficiency of the radio resources.
To this end, the mobile communication system includes various operating states according to the channel assignment and the existence/non-existence of state information. FIG. 7 illustrates a state transition diagram of a mobile communication system for the various operating states describing the packet service. As shown in FIG. 7, the state transition diagram for the packet service illustrates a packet null state, an initialization state, an active state, a control hold state, a suspended state, a dormant state and a reconnect state. In the control hold, active and suspended states, a service option is connected and in the other states, the service option is not connected.
In a conventional CDMA mobile communication system which mainly supports the voice service, a traffic channel is released upon completion of data transmission and the traffic channel is then reconnected when it is required to transmit data. The conventional channel assignment method, however, is not suitable for a packet data service because of a time delay for reconnecting the channel. Therefore, to provide the packet data service as well as the voice service, there is required an improved channel assignment method.
In general, during the packet data service, data transmission occurs intermittently (i.e., in bursts). Therefore, a transmission duration of packet data alternates with periods of non-transmission. The mobile communication system either releases or maintains a channel in use for the periods of non-transmission. However, there are drawbacks associated with both maintaining and releasing a channel, namely, release of the channel causes an increase in service time due to a time delay for reconnection of the channel, and maintaining the channel causes a waste of the channel resources. To solve these problems, a dedicated control channel is commonly provided between a base station and a mobile station to exchange traffic channel-related control signals over the dedicated control channel for the data transmission. The traffic channel is released and only the dedicated control channel and a reverse pilot/PCB channel are maintained for the data non-transmission duration. When the dedicated control channel is not activated, only the reverse pilot/PCB channel is maintained. The reverse pilot/PCB channel is required to maintain synchronization. In this manner, the mobile communication system can prevent a waste of channel resources and rapidly reconnect the traffic channel when there is data to transmit. The operating state described above is called a control hold state (see FIG. 7). The control hold state can be divided into a normal substate and a slotted substate, as shown in FIG. 8. The normal substate refers to a state where there is no data to transmit over a traffic channel, and only a control signal is exchanged over a dedicated control channel or only the reverse pilot/PCB channel is maintained. The slotted substate refers to a state where connection of the dedicated control channel is maintained but no control signal and no reverse pilot/PCB channel is maintained to reduce power consumption of a mobile station. However, to make a transition from the slotted substate to the normal substate to restart control data transmission, resynchronization should be performed between a base station and a mobile station, since no control signal is exchanged between the base station and the mobile station in the slotted substate
However, when closed-loop power control of the reverse pilot/PCB channel is maintained, as in the case where there exists a dedicated control channel and the system stays in a data transmission state even though there is no message to transmit over the dedicated control channel in the normal substate, interference and power consumption may increase unnecessarily.
FIG. 1A illustrates a conventional base station transmitter for a conventional CDMA communication system.
With regard to forward link channels, the base station includes a pilot channel for sync acquisition and channel estimation, a forward common control channel (F-CCH) for communicating a control message in common to all the mobile stations located in a cell (or service) area of the base station, a forward dedicated control channel (F-DCCH) for exclusively communicating a control message to a specific mobile station located in the cell area of the base station, and a forward dedicated traffic channel (F-DTCH) for exclusively communicating traffic data (i.e., voice and packet data) to a specific mobile station located in the cell area of the base station. The forward dedicated control channel includes a sharable forward dedicated control channel (sharable F-DCCH) for exclusively communicating a control message to a specific mobile station on a time-division basis. The forward dedicated traffic channel includes a forward fundamental channel (F-FCH) and a forward supplemental channel (F-SCH).
Referring to FIG. 1A, demultiplexers 120, 122, 124 and 126 demultiplex corresponding channel-coded interleaved channel information to I and Q channels. Here, serial-to-parallel converters can be used for the demultiplexers 120, 122, 124 and 126. It is assumed herein that signals input to the demultiplexers 120, 122, 124 and 126 are signal-mapped signals. Mixers 110, 130, 131, 132, 133, 134, 135, 136 and 137 multiply signals output from the associated demultiplexers by orthogonal codes assigned to the corresponding channels, for signal spreading and channel separation. The orthogonally spread signals output from the mixers 130-137 are gain controlled by associated amplifiers 140-147.
Signals output from the amplifiers 140-147 and the mixer 110 are summed by summers 150 and 152 according to the I and Q channels. Since the signals applied to the summers 150 and 152 were channel separated by the orthogonal codes, the respective channel signals are orthogonal to one another. Outputs of the summers 150 and 152 are multiplied by PN (Pseudo Noise) sequences PN#I and PN#Q assigned to the base station for base station identification in a complex multiplier 160. I and Q channel signals output from the complex multiplier 160 are applied to filters 170 and 171, respectively, which bandpass filter the input signals to output bandwidth-suppressed signals. The outputs of the filters 170 and 171 are amplified by amplifiers 172 and 173. Mixers 174 and 175 multiply outputs of the amplifiers 172 and 173 by a carrier cos (2xcfx80fct) to up-convert the signals to radio frequency (RF) signals. A summer 180 sums the I and Q channel signals.
A power control command transmitted from a base station to a mobile station is divided into power-up and power-down commands and is comprised of a single bit (or symbol). The mobile station determines whether to increase or decrease transmission power according to a sign of the power control bit. In FIG. 1, the power control bit has a positive sign for the power-up command and a negative sign for the power-down command.
FIG. 1B illustrates a mobile station transmitter for a conventional CDMA communication system. With regard to reverse link channels, the mobile station includes a reverse pilot/PCB (Power Control Bit) channel for multiplexing a pilot signal for sync acquisition and channel estimation and a forward power control bit for forward power control, a reverse dedicated control channel (R-DCCH) for exclusively communicating a control message to a base station, in a cell area of which the mobile station is located, and a reverse dedicated traffic channel (R-DTCH) for exclusively communicating traffic data to the base station. Further, the reverse dedicated traffic channel includes a reverse fundamental channel (R-FCH) and a reverse supplemental channel (R-SCH).
A multiplexer 210 multiplexes a signal on the reverse pilot channel and a power control bit for controlling power of the forward link. Mixers 220, 230, 240, 250 and 260 multiply corresponding channel-coded interleaved signals received over the respective reverse channels by orthogonal codes assigned to the corresponding channels to generate orthogonally spread signals for the respective channels. Outputs of the mixers 220, 240, 250 and 260 are gain controlled by amplifiers 222, 242, 252 and 262, respectively. A summer 224 sums outputs of the amplifiers 222 and 242 and an output of the multiplier 230, and a summer 254 sums outputs of the amplifiers 252 and 262. Since the signals applied to the summers 224 and 254 were channel separated by the orthogonal codes, the respective channel signals are orthogonal to one another. A complex spreader (or complex multiplier) 160 multiplies signals output from the summers 224 and 254 by a spreading code assigned to the mobile station to spread the signals. The spreading code assigned to the mobile station is generated by mixing a PN sequence for a base station, in the cell area of which the mobile station is located, by a unique long code for the mobile station. Filters 170 and 171 filter I and Q channel signals output from the complex spreader 160, respectively, to generate bandwidth suppressed signals. Amplifiers 172 and 173 amplify outputs of the filters 170 and 171, respectively. Mixers 174 and 175 multiply signals output from the amplifiers 172 and 173 by a carrier cos (2xcfx80fct) to up-convert the transmission signals to RF signals. A summer 180 sums the I and Q channel signals output from the mixers 174 and 175.
In the control hold state of the conventional CDMA communication system, a dedicated traffic channel is released and a control signal is communicated over forward and reverse dedicated control channel. A description will be provided regarding the operation of a reverse pilot/PCB channel in the control hold state. Herein, it is assumed that the control hold state is divided into a normal substate and a slotted substate. However, even in the case where the control hold state is not divided into the normal substate and the slotted substate, the reverse pilot/PCB channel will have the same operation.
A description will now be made to transmission signal structures of a base station and a mobile station according to the prior art.
Reference numeral 300 in FIGS. 3A and 3B illustrates how a mobile station conventionally transmits a signal on a reverse pilot/PCB channel, when a reverse dedicated control channel (R-DCCH) is not activated in a control hold state/normal substate. To avoid resync acquisition at a base station, the mobile station continuously transmits the reverse pilot/PCB channel in the control hold state/normal substate, and a reference value xcex81 used for closed loop power control is maintained to be equal to that in an active state unless the reference value is changed due to outer loop power control which depends on a frame error ratio. Transmission of the reverse pilot/PCB channel is discontinued when a transition to the control hold state/slotted substate occurs. However, the reverse pilot/PCB channel is transmitted before the transition, thus increasing interference of the reverse link. The increase in interference of the reverse link inevitably decreases a capacity of the reverse link.
Reference numeral 400 in FIG. 4A represents positions where a reverse dedicated control channel (R-DCCH) having a frame length of 5 ms can be conventionally generated when a reverse dedicated MAC logical channel (dmch) is generated in the control hold state/normal substate. After generating the dmch, the R-DCCH can be transmitted within 5 ms in maximum. Since the R-DCCH can be transmitted only at positions corresponding to multiples of 5 ms, the number of cases where the R-DCCH can exist is small, so that the base station need only determine the existence/nonexistence of the R-DCCH at four places in one frame. However, after generation of the dmch and until transmission of the R-DCCH, an average time delay of 2.5 ms generally occurs, which is xc2xd of the R-DCCH frame length. Here, to avoid a resync acquisition process at the base station, the mobile station continuously transmits a reverse pilot/PCB channel in the control hold state/normal substate, and a reference value xcex81 used for closed loop power control is maintained to be equal to that in an active state unless the reference value is changed due to outer loop power control which depends on a frame error ratio.
Reference numeral 410 in FIG. 4C represents a case where an R-DCCH is conventionally transmitted within 1.25 ms in maximum after generation of the dmch in the control hold station/normal substate. After generating the dmch, the R-DCCH can be transmitted within 5 ms in maximum. Here, after generation of the dmch and until transmission of the R-DCCH, an average delay time of 0.625 ms occurs. To avoid a resync acquisition process at the base station, the mobile station continuously transmits a reverse pilot/PCB channel in the control hold state/normal substate, and a reference value xcex81 used for closed loop power control is maintained to be equal to that in an active state unless the reference value is changed due to outer loop power control which depends on a frame error ratio.
Reference numerals 500 and 510 of FIG. 5A illustrate a conventional power control method for a reverse pilot/PCB channel when an R-DCCH is not activated in a control hold state/normal substate. For both the forward and reverse links, closed loop power control is performed at the same time periods. Here, to avoid a resync acquisition process at the base station, the mobile station continuously transmits a reverse pilot/PCB channel in the control hold state/normal substate, and a reference value xcex81 used for closed loop power control is maintained to be equal to that in an active state unless the reference value is changed due to outer loop power control which depends on a frame error ratio.
Reference numerals 600 and 610 of FIG. 6A represent a conventional power control method for a reverse pilot/PCB channel when an R-DCCH is activated in the control hold state/normal substate, in the case where the R-DCCH indicated by reference numeral exists every 5 ms in a 20 ms basic frame without overlapping. For both the forward and reverse links, closed loop power control is performed at the same time periods. Here, to avoid a resync acquisition process at the base station, the mobile station continuously transmits a reverse pilot/PCB channel in the control hold state/normal substate, and a reference value xcex81 used for closed loop power control is maintained to be equal to that in an active state unless the reference value is changed due to outer loop power control which depends on a frame error ratio.
As stated above, the conventional method of maintaining a reference value for closed loop power control for the reverse pilot/PCB channel in the control hold state/normal substate is advantageous in that the base station can avoid the resync acquisition procedure and make a fast transition to an active state. However, the conventional method increases interference to the reverse link, causing a reduction in capacity of the reverse link. In addition, for the forward link, closed loop power control is performed at the same speed (or rate) as in the active state, thus causing an increase in interference of the forward link and a decrease in capacity of the forward link due to the reverse power control bits.
It is, therefore, an object of the present invention to provide a communication device and method for changing a reference value for reverse closed loop power control in a control hold state in a CDMA mobile communication system.
It is another object of the present invention to provide a communication device and method for changing a reverse power control rate in a control hold state in a CDMA mobile communication system.
It is further another object of the present invention to provide a communication device and method for intermittently transmitting a power control command for controlling reverse transmission power in a control hold state in a CDMA mobile communication system.
To achieve the above objects, a base station device for a CDMA communication system includes a controller for changing a reference value for reverse closed loop power control in a control hold state, and a forward dedicated control channel transmitter for transmitting a power control bit for controlling transmission power of a reverse link according to the changed power control reference value.